In electrophotographic image forming apparatuses such as a multifunction printer, a copying machine, a printer, and a facsimile, a developing process is carried out by providing developer to a latent image formed on a surface of a photoreceptor. Various methods are available as a developing process. Among them, a magnetic brush developing method by using two-component developer composed of toner and magnetic carrier has been widely used because it is excellent in high-speed capability. Note that two-component developer is merely called developer and magnetic carrier is called carrier hereinafter.
A developing device in a magnetic brush developing method includes a developer tank and a magnetic developing roller, as illustrated in FIG. 1 in the following Patent Citation 1. The developing roller magnetically attracts (scoops) developer stored inside the developer tank, and holds the developer on an external surface of the developing roller, and then conveys the developer to a photoreceptor. This procedure achieves the developing process.
Recently, minimization of the particle size in toner and carrier, both of which are contained in developer, is positively carried out to improve image quality. However, as particle sizes of toner and carrier are minimized, flowability of developer drops, resulting in a problem such as decline in image density because the developing roller fails to attract enough developer from the developer tank. An example of a technique to solve this problem is a developing device in the Patent Citation 1. In the Patent Citation 1, a developing device includes a developing roller composed of: a developing sleeve (axis sleeve) with a plurality of grooves on an outer surface that extend in a direction of a rotation axis; and a magnet roller inserted in a hollow inside the developing sleeve, and the distance between the grooves and the depth of each groove are set to be within predetermined ranges, in order to improve the attracting effect (scooping effect) of developer by the developing roller.
[Patent Citation 1] Japanese Unexamined Patent Publication, Tokukai 2004-170555 (date of publication: Jun. 17, 2004)
However, in a developing device, which includes a developing sleeve having a plurality of grooves on the outer surface, the following adverse effect was found. After printing an image partially having an area with extremely high density, another image printed at a given density partially has an area with lower density than the given density.
The following explains this adverse effect. In a magnetic brush developing method, normally, a bias is applied to a developing roller during a developing process. This allows a potential of developer on the external surface of the developing sleeve to be substantially uniform.
When there is performed first printing in which an image partially having an area with extremely high density (central part in the main scanning direction in FIG. 1 (a)) is printed as shown by reference number 300 in FIG. 1(a) and then the developing device is driven, an electric potential difference of developer is generated on the outer surface of the developing sleeve 200 between (i) area “α” which is a central part in the main scanning direction (corresponding to the area with extremely high density in the first printing) and (ii) area “b” which is a part other than the area “α”, as illustrated in FIG. 1(b). Specifically, an electric potential of developer on the area “α” corresponding to the area with extremely high density in the first printing is lower than that of the developer on the area “b”. Note that such phenomenon of potential differences is called “development memory” in this specification.
Assume that after the development memory illustrated in FIG. 1(b) is generated on the developing sleeve 200 by carrying out the first printing, for example, there is performed second printing in which a solid image having substantially uniform density. In this case, the potential of developer on the area “α” is lower than the potential of developer on the area “b”. Therefore, in a photosensitive drum, a potential of a part having toner provided from the area “α” is lower than that of a part having toner provided from the area “b”. As a result, in an image transferred on a sheet, an area printed with the toner provided from the area “α” has lower density than the area printed with the toner provided from the area “b”. Namely, in the case of printing a solid image having substantially uniform density by the developing sleeve 200 in which the development memory is generated as illustrated in FIG. 1 (b), a central part in the main scanning direction has lower density than areas on both sides of the central part, as shown by a reference number 400 in FIG. 1 (a).
As explained above, in the case where an image partially having an area with extremely high density is printed, the development memory is generated in the developing sleeve. Thereafter, when an image is printed at a given density, the image partially has an area with lower density than the given density due to the development memory. Therefore, it is apparent that unless the generation of the development memory is prevented, density unevenness is generated, resulting in the degradation of the printed image.